Radio communication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually network architecture will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructure and would, therefore, typically prefer to migrate gradually to all-IP network architectures to allow them to extract sufficient value from their investment in existing infrastructures. In order to provide the capabilities needed to support next generation radio communication applications, while at the same time using legacy infrastructure, network operators will deploy hybrid networks wherein a next generation radio communication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network.
One example of such a hybrid network involves an existing second generation (2G) radio communication system, such as the Global System for Mobile communication (GSM), onto which a next generation “long term evolution” (LTE)/“system architecture evolution” (SAE) system is overlaid. As will be appreciated by those skilled in the art, GSM systems have been modified and updated over time. For example, GSM release 1997 added packet data capabilities using General Packet Radio Service (GPRS) and GSM release 1999 introduced higher speed data transmissions through a system called Enhanced Data Rates for GSM Evolution (EDGE). Although not yet standardized, LTE systems will ultimately be designed in accordance with a new version of the UMTS standards, see, e.g., 3GPP TR 25.913 available online at www.3gpp.org. Target performance goals for LTE systems currently include, for example, support for 200 active calls per 5 MHz cell and less than 5 ms latency for small IP packets.
When an LTE/SAE system is overlaid onto or next to a GSM system, various types of inter-system interoperability will become desirable, one of which is handoff or handover. Inter-system handoff refers to, for example, the process whereby a mobile unit, e.g., a cellular telephone, wireless PDA or laptop, which is currently being supported by a first radio communication system is transferred to the support of a second radio communication system. In the context of this application, as shown conceptually in FIG. 1, an inter-system handoff of interest involves the transfer of communication support of mobile station (MS) 102 from an LTE radio access network 30 to GSM radio access network 20 or vice versa. Such handoffs may be performed for a variety of reasons. For example, a mobile station 102 which is currently being served by the LTE radio access network 30 may have moved into a geographic area wherein it can be better served by the GSM radio access network 20. Alternatively, the inter-system handoff may be performed to provide load balancing between the GSM radio access network 20 and the LTE radio access network 30.
Regardless of the particular reason for the handoff, various signaling needs to be performed in order to complete the transfer of support responsibility for the mobile station 102 from or to the LTE radio access network 30. Additionally, unless the mobile station 102 can simultaneously receive signals from both the GSM radio access network 20 and the LTE radio access network 30, the inter-system handoff will be a hard (“break before make”) handoff rather than a soft (“make before break”) handoff. As an example, handover between GSM (2G) and UMTS (3G) is always a hard handoff. For hard handoffs it is further desirable to minimize the intervening time interval during which the mobile station 102 is not connected to either the GSM radio access network 20 or the LTE radio access network 30. Accordingly the exemplary embodiments described herein address the need for mechanisms to facilitate such inter-system handoffs.